Category

Published on

11 Nov 2011

Abstract

Earthquake engineers take advantage of inertial forces to protect a building from damage during an earthquake. By using base isolation to separate the building from the ground, they can reduce the movement of the floors and the building contents. In this lab, participants observe how base isolation protects buildings during an earthquake using horizontal rollers to model base isolators and the masses on rods to model a building.

Introduction

Why do you need to wear a seat belt? If you are in a car going 70 mph and slam on the brakes, you will continue to go at 70 mph (and through the windshield!) unless something like a seat belt stops you.

Newton's first law states that a body in motion will stay in motion and a body at rest will stay at rest unless acted on by an outside force. This is called inertia. It is the tendency of something to stay the way it is.

The law of inertia is important when talking about buildings in an earthquake. A building can be thought of as a large mass, and according to the law of inertia, it wants to stay at rest and remain motionless unless acted on by an outside force. In an earthquake, the bottom parts of the building move and the upper parts of the building don't because of inertia. This is called inertial force. This puts a lot of stress on the parts that make up the building. It is this inertial force that engineers have to try and minimize when designing buildings.

One of the ways that earthquake engineers protect a building is to use the inertia of the building to their advantage. If they can keep the body from moving, then the top floor won't move either! So, if an engineer can find a way to keep the earthquake from acting on the building, it won't move. Base isolation is separating the building from the ground so that the earthquake can't affect it.

If you lay a toy car or a skate on a cardboard sheet and yank the cardboard back and forth like the horizontal motions of an earthquake, what happens? The car will not slide as much as the cardboard, but it will still move slightly back and forth. Those wheels on the car and skate "isolated" the top part from the earthquake!

But how do we do that with a big building? In reality, engineers don't use big wheels. Instead, they use a special material between the columns of the building and its foundation. This supports the building so that it can stand, but it lets the "ground" move from side-to-side underneath it.

Movement of Building with Base Isolation vs. No Base Isolation

earthscience.org/geopro/seismic/seismic.html

In the pictures above, the building on the left (fixed base) has no base isolation. If you were sitting on the top floor of that building, look how much you'd move in an earthquake! The building on the right, however, has base isolation. When an earthquake comes, the base isolation bearings move, and not the building, so the people on the top floor don't move.

Once we isolate the structure of the building from the earth, we must make sure that the building does not move around too much, and that it goes back to its original position. Engineers use rubber pads to base isolate buildings, and they add into the rubber pads special fillers that increase the pads' friction. This helps lessen the back and forth motion of the building. Friction also absorbs some of the earthquake energy that would otherwise go into shaking the building and reduces the quake's impact on the structure. (Remember that energy never disappears completely but only changes from one form to another and that energy absorbed by friction gets changed into harmless heat.)

Another bonus to adding fillers to our rubber pads is that they offer the advantage of preventing, through friction, the frequent and annoying small oscillations (back-and-forth movements) that even a light wind or a car driving by would have on a building or a bridge built on entirely rubber pads.

Rubber Pads Used in Base Isolation

earthscience.org/geopro/seismic/seismic.html

Earthquake Engineering Component

Buildings on base isolators experience significantly less motion. This can be very important in protecting valuable contents such as the artwork in a museum or essential life support equipment in a hospital. Examples of base isolated buildings are Los Angeles City Hall, San Francisco Asian Art Museum, and the USC Hospital. The San Francisco Airport International Terminal is one of the largest base isolated structures in the world (http://www.celebratingeqsafety.com/sfo). Many photos of base isolated buildings can be found at http://www.dis-inc.com/portfolio.html

This 2 minute video shows a room filled with furniture as it shakes on base isolation and without isolation. It also shows a base isolator being tested in a laboratory http://www.youtube.com/watch?v=phgdkqn9aTI

Learning Objectives and Standards

Links to the National Science Standards and to individual State Science Standards are available by using this link:

Material List

Shake table

Base isolation attachment

Masses on rods

Procedure

Even though real buildings use rubber pads as base isolators, we can experimentally look at base isolation by using a set of rollers. In this lab, horizontal rollers will be used as a base isolator and the masses on rods will be used as a building.

Attach the horizontal rollers to the shake table using the two screws.

Test that the rollers are attached to the shake table by trying to shake it with your hands.

Make sure that the safety stops are attached and in position so that the masses on rods will not come off the table.

Place the masses on rods plate on the horizontal rollers.

Start the shake table and allow it to calibrate using the procedure outlined in the shake table operations manual.

Assessment

Collecting and Analyzing Data Key:

What happened to the masses on rods when the base isolator is in place?The masses on rods didn't move.

Why do we need a space between the edges of the base isolator and the sides of the masses on rods table?If the masses on rods table touches the shake table, it is no longer "isolated" by the base and is therefore subjected to all of the forces of the earthquake.

Conclusion Key:

Why is it important to allow the ground to move underneath the building? By allowing the ground to move underneath the building, the building remains relatively motionless which means that the structural components are not stressed as much as if it were moving with the ground.

What would happen if the base of the building moves too much? If the building's base moves too much it can run into other structures on the ground, such as retaining walls, entry steps, or even a perimeter moat. This can cause damage to the building and other structures which is called pounding.

Would base isolators be able to protect a building if the ground moves up and down? No, base isolators are only able to protect a building if the ground is moving horizontally (side to side)